This invention relates to silver halide photographic materials, more particularly to high-sensitivity, low-fog silver halide photographic materials.
The essential requirements that have to be satisfied by silver halide photographic materials are that they have high sensitivity while suffering from low fog. The common practice for meeting this need is to control in various ways the conditions of growing silver halide grains to be contained in silver halide emulsions for use in silver halide photographic materials. The step involved in the growth of silver halide grains for modifying their size and crystallographic shape to thereby establish the grain size and distribution is generally referred to as "physical ripening".
Physical ripening is divided into two typical types on the basis of the driving force that causes grain growth. In the first type of physical ripening, silver halide (hereinafter abbreviated to AgX as appropriate) grains are grown on the basis of variations in dissolving pressure among fine grains. Stated more specifically, new irregular shaped micrograins of AgX are created in a suspension medium and, in accordance with variations in site dependent and/or time-dependent conditions of grain formation in the suspension medium during the creation of said new micrograins, the relative sizes of the micrograins will vary to cause differences in their tendency to agglomerate or the dissolving pressures of individual micrograins that are determined from their solubility products and sizes will vary, causing those grains to grow either by agglomeration or by survival of grains having lower dissolving pressures at the expense of those having higher pressures which will disappear eventually. As the surface activity of agglomeration is exhausted, grain growth by the difference in dissolving pressure will dominante. For the purpose of the present discussion, this process of grain growth may conveniently be called a "dissolving pressure dependent process". The growth process that is solely dependent on dissolving pressure is especially referred to as "Ostwald ripening". Taken individually, the AgX crystal grains produced by this process are subject to annealing and, as a result of the gradual decrease in dislocations and other crystal defects, a normal equilibrium will finally be reached. However, taken collectively (from an inter-grain viewpoint), the grain size distribution will broaden unavoidably and it is difficult to guarantee uniformity in the compositional ratios of AgX contained in the grains and their crystallographic shape.
The second type of physical ripening is the process in which existing irregular shaped micrograins or crystal grains in a suspension medium are used as nuclei for crystal growth onto the surfaces of which new AgX is deposited or precipitated to cover and increase the size of the existing grains. This process may be referred to as a "crystal nucleation process", in which the growth of grains is inevitably accomplished by supplying new AgX in multiple stages. According to this crystal nucleation process, not only grains already having stable sizes but also the initially created primitive micrograins work as nuclei for the crystal growth of subsequently created AgX if pAg, the rate of creation or pH is properly adjusted, whereby a group of crystalline grains grown to a substantially uniform size can be obtained. Further, a crystal control agent may be additionally used during the period when new AgX is supplied in multiple stages and this is advantageous for imparting a desired crystallographic shape. If the suspension of primitive micrograins initially formed in the "crystal nucleation process" is designated as a primitive (zero-order) seed emulsion, subsequent suspensions can accordingly be defined as the first-order, second-order and third-order up to the nth-order seed emulsions in succession.
Needless to say, a method intermediate between the two typical processes may be adopted as required for producing emulsions.
If, in the above-described processes for the production of emulsions, any by-products of reaction or excess compounds or additives that will eventually dissolve in emulsions are anticipated to cause adverse effects on subsequent steps or characteristic designs, those deleterious materials are usually eliminated from the system. In modern emulsion making technology, the flocculation process which employs flocculants is commonly selected as a means of eliminating the deleterious materials.
While there are two typical processes available for physical ripening, the dissolving pressure dependent process has the disadvantage of producing a broad size distribution of AgX grains, inter-grain variations in the compositional ratios of incorporated AgX, and differences in the conditions of grain surfaces, so the emulsions prepared by this process are variable in light acceptance, quantum efficiency, the adsorbability of additives, and developing ability, with the result that the finished emulsions vary greatly in performance.
On the other hand, the crystal nucleation process is capable of producing emulsions comprised of a group of monodisperse grains that are uniform in grain size and compositional ratio between grains and that feature easy control of characteristics and hence consistent production of emulsions having desired characteristics. Therefore, this process is extensively used today to meet the increasingly stringent requirements of photographic performance.
However, the emulsions prepared by the conventional crystal nucleation process in which crystals are grown using a seed emulsion as in the prior art have the disadvantage that they do not necessarily satisfy the need of users for higher sensitivity and lower fog.